The present invention relates to a boron-phosphide-based semiconductor light-emitting device including a semiconductor substrate, a boron-phosphide-based semiconductor layer formed on the substrate, and an active layer formed of a Group III-V compound semiconductor. Particularly, the present invention relates to a boron-phosphide-based semiconductor light-emitting device which has an improved electrode structure and exhibits high emission intensity.
Conventionally, techniques for producing semiconductor light-emitting devices (e.g., a light-emitting diode (LED) and a laser diode (LD)) incorporating a layer formed of boron phosphide (BP), which is a Group III-V compound semiconductor (see U.S. Pat. No. 6,069,021) have been proposed. A conventional boron-phosphide-based semiconductor light-emitting device has a stacked layer structure including a substrate formed of, for example, single-crystal silicon, and a boron phosphide layer which is formed on the substrate and serves as a buffer layer (see U.S. Pat. No. 6,069,021). Recently, a stacked layer structure employed for producing semiconductor light-emitting devices, which includes a light-emitting portion of a pn-junction-type double hetero structure including a wide-band-gap boron phosphide layer serving as a cladding layer (see Japanese Patent Application No. 2001-158282) has been proposed.
A conventional boron-phosphide-based semiconductor light-emitting device incorporating an electrically conductive silicon substrate includes a bottom electrode provided on the bottom surface of the silicon substrate, and a top electrode provided on a top layer constituting a stacked layer structure similar to that described above. The top electrode or bottom electrode is formed of a metallic material capable of establishing ohmic contact with an n-type or p-type semiconductor layer on which the electrode is provided (see Japanese Patent Application Laid-Open (kokai) No. 2-275682). Regarding the boron-phosphide-based semiconductor light-emitting device incorporating the silicon substrate, a technique for forming a top positive electrode on a contact layer formed of a p-type conductive boron phosphide layer has been proposed (see Japanese Patent Application Laid-Open (kokai) No. 2-275682).
In a light-emitting device such as an LED or an LD, a top electrode generally has an electrical polarity opposite that of a bottom electrode, and a negative electrode is provided on an n-type semiconductor layer and formed of a metal capable of establishing ohmic contact with the n-type semiconductor layer. In a conventional boron-phosphide-based semiconductor light-emitting device, a portion of an electrode that is in contact with a p-type or n-type semiconductor layer consists of a metal capable of establishing ohmic contact with the p-type or n-type semiconductor layer. In a boron-phosphide-based semiconductor device (particularly, a light-emitting device) including an electrically conductive silicon substrate, when a portion of an electrode that is in contact with a p-type or n-type semiconductor layer is formed of a metal capable of establishing ohmic contact with the semiconductor layer, current for operating the light-emitting device is advantageously caused to flow between top and bottom electrodes.
However, in a boron-phosphide-based semiconductor light-emitting device, when a semiconductor layer on which a top electrode is formed has a thickness and a resistivity such that operation current is insufficiently diffused therein in a horizontal direction, the operation current preferentially flows to a region of a light-emitting portion that is located just below the top electrode, and the operation current fails to be diffused throughout the light-emitting portion. That is, when operation current is caused to flow, light is emitted from merely the region of the light-emitting portion that is located just below the top electrode. In addition, since light emitted from the region located just below the top electrode is shielded by the electrode, difficulty is encountered in extracting the emitted light from the device. Therefore, the boron-phosphide-based semiconductor light-emitting device which exhibits high luminance can be hardly obtained.
An object of the present invention is to provide a boron-phosphide-based semiconductor light-emitting device (LED) including a boron-phosphide-based semiconductor layer serving as a cladding layer, and a top electrode having a structure such that the operation current can be caused to flow to a wide region of a light-emitting portion from which light is advantageously emitted, not just to a region located below the top electrode.
Accordingly, the above object of the present invention has been achieved by providing the following.
(1) A boron-phosphide-based semiconductor light-emitting device comprising a semiconductor substrate of a first conduction type having, on its bottom surface, a bottom electrode; a first boron-phosphide-based semiconductor layer of a first conductive type provided on the substrate; a Group III-V compound semiconductor active layer provided on the first boron-phosphide-based semiconductor layer; a second boron-phosphide-based semiconductor layer of a second conduction type provided on the active layer; and a top electrode provided on the surface of the second boron-phosphide-based semiconductor layer, wherein the top electrode includes a lower electrode and an upper electrode, the lower electrode is in direct contact with the second boron-phosphide-based semiconductor layer and formed of a metal incapable of establishing ohmic contact with the second boron-phosphide-based semiconductor layer, and the upper electrode is provided on the lower electrode and formed of a metal capable of establishing ohmic contact with the second boron-phosphide-based semiconductor layer, wherein a portion of the upper electrode is in contact with the surface of the second boron-phosphide-based semiconductor layer.
(2) A boron-phosphide-based semiconductor light-emitting device according to (1) above, wherein the first boron-phosphide-based semiconductor layer serves as a cladding layer for the active layer.
(3) A boron-phosphide-based semiconductor light-emitting device according to (1) or (2) above, which further comprises a buffer layer formed of an amorphous or polycrystalline boron-phosphide-based semiconductor layer provided between the semiconductor substrate and the first boron-phosphide-based semiconductor layer.
(4) A boron-phosphide-based semiconductor light-emitting device according to any one of (1) through (3) above, wherein the first boron-phosphide-based semiconductor layer, the active layer, and the second boron-phosphide-based semiconductor layer constitute a light-emitting portion of double hetero (DH) structure.
(5) A boron-phosphide-based semiconductor light-emitting device according to any one of (1) through (4) above, wherein the second boron-phosphide-based semiconductor layer is formed of p-type boron phosphide (BP), and the lower electrode is formed of a gold.germanium (Au.Ge) alloy, a gold.indium (Au.In) alloy, a gold.tin (Au.Sn) alloy, or an indium.tin composite oxide (ITO).
(6) A boron-phosphide-based semiconductor light-emitting device according to (5) above, wherein the upper electrode is formed of a gold.zinc (Au.Zn) alloy or a gold.beryllium (Au.Be) alloy.
(7) A boron-phosphide-based semiconductor light-emitting device according to any one of (1) through (4) above, wherein the second boron-phosphide-based semiconductor layer is formed of n-type BP, and the lower electrode is formed of a gold.zinc (Au.Zn) alloy or a gold.beryllium (Au.Be) alloy.
(8) A boron-phosphide-based semiconductor light-emitting device according to (7) above, wherein the upper electrode is formed of a gold.germanium (Au.Ge) alloy, a gold.indium (Au.In) alloy, or a gold.tin (Au.Sn) alloy.
(9) A boron-phosphide-based semiconductor light-emitting device according to any one of (1) through (4) above, wherein the lower electrode and the second boron-phosphide-based semiconductor layer are capable of establishing a Schottky junction.
(10) A boron-phosphide-based semiconductor light-emitting device according to (9) above, wherein the lower electrode is formed of nickel (Ni), tungsten (W), tantalum (Ta), aluminum (Al), titanium (Ti), gold (Au), titanium nitride (TiN), or tungsten carbide (WC).
The present invention also provides the following.
(11) A method for producing a boron-phosphide-based semiconductor light-emitting device comprising successively forming, on a semiconductor substrate of a first conduction type, a first boron-phosphide-based semiconductor layer of a first conduction type, a Group III-V compound semiconductor active layer, and a second boron-phosphide-based semiconductor layer of a second conduction type; forming a bottom electrode on the bottom surface of the semiconductor substrate; and forming a top electrode on the surface of the second boron-phosphide-based semiconductor layer, wherein a lower electrode containing a metal incapable of establishing ohmic contact with the second boron-phosphide-based semiconductor layer is formed to come into direct contact with the second boron-phosphide-based semiconductor layer, and subsequently an upper electrode containing a metal capable of establishing ohmic contact with the second boron-phosphide-based semiconductor layer is formed on the lower electrode such that a portion of the upper electrode comes into contact with the surface of the second boron-phosphide-based semiconductor layer.
(12) A method for producing a boron-phosphide-based semiconductor light-emitting device according to (11) above, wherein the first boron-phosphide-based semiconductor layer serves as a cladding layer for the active layer.
(13) A method for producing a boron-phosphide-based semiconductor light-emitting device according to (11) or (12) above, which further comprises forming, between the semiconductor substrate and the first boron-phosphide-based semiconductor layer, a buffer layer containing an amorphous or polycrystalline boron-phosphide-based semiconductor layer.
(14) A method for producing a boron-phosphide-based semiconductor light-emitting device according to any one of (11) through (13) above, wherein the first boron-phosphide-based semiconductor layer, the active layer, and the second boron-phosphide-based semiconductor layer constitute a light-emitting portion of double hetero (DH) structure.
The above object of the present invention has also been achieved by providing the following.
(15) A light-emitting diode comprising a boron-phosphide-based semiconductor light-emitting device as recited in any one of (1) through (10) above.